This invention relates to memory accessing techniques in a microcomputer system and more particularly to a memory accessing scheme compatible with both static and dynamic memory.
The working memory of a microcomputer system normally comprises semiconductor RAMS which are memory cell arrays comprising transistor like devices. Generally, these memories or RAMS are divided into two general types. Static-type RAMS have their transistor devices arranged into individual flip-flop circuits in each memory cell. Dynamic type RAMS utilize the transistor devices for charge storage, and need periodic charge refreshing to retain the data stored in them. The circuitry to provide the charge refreshing is often embodied with the dynamic RAM itself on a single chip which is then normally designated as an integrated dynamic RAM. Integrated dynamic RAMS also organize the stored data on byte boundaries as opposed to the typical bit-wide storage of the pure dynamic RAM.
Operating characteristics of static and dynamic RAMS differ to the extent that a microcomputer system designed for one type of RAM cannot readily access the other type of RAM in a working memory system. Static RAM and indeed other static memory devices are responsive to voltage levels of control signals while dynamic or integrated dynamic RAMS are normally triggered by the leading edges of control signals. These differing requirements are reflected also in the timing of control signals to access working memory. For example, an address applied to a dynamic or integrated dynamic RAM must be stable during the leading edge of a chip enable signal. The timing requirements of the static RAM and other static memories are different.
For a given memory size, the dynamic RAM requires less space and power and in general is less expensive than the equivalent size static RAM. However, the dynamic RAM requires a fixed amount of additional interface circuitry (e.g., refresh circuitry) not required by static RAMS. Therefore, the dynamic RAM is more cost effective at large memory sizes and the static RAM excels at smaller memory sizes with an overlap wherein the static RAM competes with the integrated dynamic RAM. Generally, the development of static RAM chips of a given memory capacity lags behind the development of similar sized dynamic RAMS. So if a microcomputer system has fairly large memory requirements or space and power limitations and is within the overlap memory range, a working store comprised of integrated dynamic RAMS is generally specified. Subsequently, at a later state of memory development as static RAMS and memories of larger capacity become generally available, it may be desired to substitute static RAMS for the dynamic RAMS if they have become cost advantageous and have the required memory capacity and because of their simpler interfacing and operational control signal requirements. However, the substitution may not be feasible since the microprocessor or other aspects of the microcomputer system are not designed to accommodate static RAM. The need for redesign may dictate that the system continue to use the integrated dynamic RAMS initially specified when, in fact, the static RAM has become the preferred embodiment for the working memory.